Question

Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this redox change taking place in water.

Answer 1

The given redox reaction can be represented as:
$Cl_2(s)+SO_2(aq)+H_2O\rightarrow Cl^−(aq)+SO^{2−}_4(aq)$

The oxidation half reaction is:
$\overset{+4}SO_2(aq)\rightarrow \overset{+6}SO_4^{2−}(aq)$

The oxidation number is balanced by adding two electrons as:
$SO_2(aq)\rightarrow SO_4^{2−}(aq)+2e^−$

The charge is balanced by adding $4H ^+$ ions as:
$SO_2(aq)\rightarrow SO_4^{2−}(aq)+4H^+(aq)+2e^−$

The $O$ atoms and $H ^+$ ions are balanced by adding $2H_2O$ molecules as:
$SO_2(aq)+2H_2O(l)\rightarrow SO_4^{2−}(aq)+4H^+(aq)+2e^−$ ………(i)

The reduction half reaction is:
$Cl_2(s)\rightarrow Cl^−(aq)$

The chlorine atoms are balanced as:
$\overset{0}Cl_2(s)\rightarrow \overset{-1}Cl^−(aq)$

The oxidation number is balanced by adding electrons
$Cl_2(s)+2e^−\rightarrow 2Cl^ -(aq)$… (ii)
The balanced chemical equation can be obtained by adding equation (i) and (ii) as:
$Cl_2(s)+SO_2(aq)+2H_2O(l)\rightarrow2Cl^−(aq)+SO_4^{2−}(aq)+4H^+(aq)$